An ultrasonic sensor disclosed in U.S. 2006/0196272 corresponding to JP-A-2006-242650 includes one transmitting device and four receiving devices. When an alternating voltage is applied to an electrode film on a membrane portion, the membrane portion and the electrode film resonate at a predetermined frequency so that the transmitting device transmits an ultrasonic wave. The ultrasonic wave is reflected from an object to be detected and received by each receiving device. The receiving devices output signals corresponding to the received ultrasonic waves. A distance and an angle of the object relative to the ultrasonic sensor is calculated based on differences in time and phase between the output signals of the receiving devices.
By the way, when an ambient humidity changes, the ultrasonic wave transmitted by the transmitting device is attenuated. Thus, a sound pressure of the ultrasonic wave received by the receiving devices changes with the ambient humidity. In the ultrasonic sensor disclosed in U.S. 2006/0196272, the transmitting device transmits ultrasonic waves having two different frequencies, and the humidity is calculated based on a difference in attenuation coefficient between the ultrasonic waves. The calculated humidity is used to correct a preset operating humidity. However, when the ultrasonic sensor includes an acoustic matching member, it may be difficult to accurately detect the humidity.
An ultrasonic sensor disclosed in JP-A-S63-103993 includes an acoustic matching member. The acoustic matching member has a microballoon made of glass inside so that variations in characteristics of the acoustic matching member due to a temperature change can be reduced. However, some factors such as the size and the strength of the acoustic matching member may make it difficult to place such a microballon inside the acoustic matching member.